This invention relates to silicon single crystals for use as semiconducting materials. More particularly, it is directed to a high-quality silicon single crystal that is grown by a Czochralski method (hereinafter referred to as xe2x80x9cCZ methodxe2x80x9d) and that is excellent in device characteristics, and to a method of producing such a high-quality silicon single crystal.
A variety of methods are available to grow silicon single crystals for use as semiconducting materials. Among these methods, the CZ method is extensively used.
FIG. 1 is a schematic sectional view of a single crystal producing apparatus used for producing single crystals by a normal CZ method. As shown in FIG. 1, a crucible 1 comprises a quartz-made, bottomed, cylindrical inner layer holding container 1a, and a graphite-made, similarly bottomed and cylindrical outer layer holding container 1b that is fitted over the outside of the inner layer holding container 1a. The constructed crucible 1 is supported by a support shaft 1c that is rotated at a predetermined speed. Outside the crucible 1 is set a heater 2, which is provided in the form of a concentric cylinder. The crucible 1 is charged with a melt 3 that is a raw molten material heated by the heater 2. A pulling shaft 4, such as a pull rod or a wire, is provided at the center of the crucible 1. A seed chuck and a seed crystal 5 are attached to the distal end of the pulling shaft 4, and the seed crystal 5 is brought into contact with the surface of the melt 3 in order to grow a single crystal 6. Further, by pulling the seed crystal 5 at a predetermined rate using the pulling shaft 4 while rotating the pulling shaft 4 in a direction opposite to the direction of the crucible 1 rotated by the support shaft 1c, the melt 3 is solidified at the distal end of the seed crystal 5, thereby gradually growing the single crystal 6.
For single crystal growth, a seed-constricting step is carried out first so as to make the crystal dislocation-free. Thereafter, to secure a body diameter of the single crystal, a shoulder is formed, and when the body diameter has been obtained, a shoulder-changing step is performed. Then, the single crystal growing process is shifted to the single crystal body-growing step while maintaining the obtained body diameter. When the single crystal has been grown to a predetermined length while maintaining the body diameter, a tail constricting step is carried out so as to separate the single crystal from the melt in the dislocation-free state. Thereafter, the single crystal separated from the melt is taken out of the puller, and cooled under a predetermined condition, and processed into wafers. The wafers thus processed from the single crystal are used as substrate materials for the preparation of various devices.
In an in-plane area of a wafer that is processed through the above-described steps, there may occur, in some cases, oxidation-induced stacking faults (hereinafter referred to as xe2x80x9cOSFxe2x80x9d) as defects appearing through heat treatments. Ring-like extending OSF (hereinafter referred to as xe2x80x9cR-OSFxe2x80x9d) may appear in some cases depending on the pulling condition of a single crystal. At the same time, there occur, in the in-plane area of the wafer, defects that called xe2x80x9cgrown-in defects.xe2x80x9d These grown-in defects are formed during single crystal growth and detected in wafers subjected to heat treatments or predetermined evaluation processes.
FIG. 2 schematically illustrates a generally observed relationship between the pulling rate during single crystal growth and the positions where crystal defects occur. As shown in FIG. 2, in a silicon single crystal grown by the CZ method, the region where R-OSF appear shrinks inward from the outer edge of the crystal as the pulling rate is decreased. Therefore, when a single crystal is grown fast, the crystal in the inner region of R-OSF expands into the whole wafer, while when a single crystal is grown slowly, the crystal in the outer region of R-OSF expands into the whole wafer.
Grown-in defects observed on a surface of a wafer are different between a rapidly grown crystal and a slowly grown crystal. In the crystal that is grown fast, i.e., in the inner region of R-OSF, defects called xe2x80x9claser scattering tomography defectsxe2x80x9d (they are also called as xe2x80x9cCOPxe2x80x9d and xe2x80x9cFPD,xe2x80x9d and are detected by different evaluation methods, but are derived from the same kind of defect) are detected. On the other hand, in the crystal that is grown slowly, i.e., in the outer region of R-OSF, defects called xe2x80x9cdislocation clustersxe2x80x9d are detected.
FIG. 3 schematically illustrates an example of a typical distribution of defects observed at an in-plane position A of the crystal of FIG. 2 previously described. This schematically shows the results of observations made through X-ray topography as to the distribution of defects of a wafer after the wafer was sliced from a single crystal immediately after growth, had Cu deposited thereon while immersed into an aqueous solution of copper nitrate, and heat-treated for 20 minutes at 900xc2x0 C. That is, in the in-plane area of the wafer, R-OSF appears at a position that is about ⅔ of the outside diameter, and laser scattering tomography defects are found inside R-OSF. Further, an oxygen precipitation-promoting region exists immediately outside R-OSF so as to touch R-OSF. Oxygen precipitates easily form in this region. Around the outer edge of the wafer extends a region where dislocation clusters easily occur. Furthermore, it is observed that a denuded zone free of dislocation clusters is slightly present immediately outside the oxygen precipitation promoting region, and a denuded zone free of laser scattering tomography defects is slightly present inside R-OSF so as to touch the ring.
OSF impair electrical properties, e.g., in the form of increased leak current while showing themselves up in a high-temperature thermal oxidation process during device fabrication, and dislocation clusters also greatly deteriorate device characteristics. Therefore, a single crystal is usually produced by adjusting the growing rate so that R-OSF is located around the outer edge of a wafer. On the other hand, laser scattering tomography defects are factors for deteriorating the initial oxide film withstand voltage characteristics, and they must also be minimized.
As described earlier, to suppress the occurrence of R-OSF on a surface of a wafer, a single crystal is usually grown under such a condition that the R-OSF position is limited within the outer edge of the wafer. However, it is known that the R-OSF position is determined, in addition to the pulling rate, by the highest temperature range (from the melting point to 1250xc2x0 C.) in which the crystal stays during growth, and is hence affected by the heat history of the crystal in the highest temperature range during pulling. Thus, to determine the R-OSF position, attention must be paid to two factors, i.e., the temperature gradients in the direction of the pulling shaft and the pulling rate, which are to be achieved while a single crystal being grown stays in the highest temperature range. That is, the R-OSF position can be limited around the outer edge of a wafer by decreasing the temperature gradients when the pulling rate is not changed, or by decreasing the pulling rate when the temperature gradients are not changed.
To check the position and width of R-OSF occurring in the in-plane area of a wafer, it is effective to observe the distribution of defects in the wafer through X-ray topography after immersing the wafer that is processed from an as-grown single crystal into an aqueous solution of copper nitrate to thereby deposit Cu thereon, and heat-treating it for 20 minutes at 900xc2x0 C. Further, the position of the previously described oxygen precipitation-promoting region present immediately outside R-OSF can also be checked through a similar method.
When a silicon single crystal has low oxygen content of, e.g., 13xc3x971017 atoms/cm3 or less, one may not observe R-OSF clearly with the above-described method in some cases. In such cases, it is suggested that a ring-like region where the amount of oxygen precipitates is small be observed through X-ray topography after a wafer processed from an as-grown single crystal is charged into a heat treatment furnace of 650xc2x0 C., thereafter heated at a rate of 8xc2x0 C./min or less, and then heat-treated for 20 hours at 900xc2x0 C. and for 10 hours at 1000xc2x0 C. Further, the position and width of the oxygen precipitation-promoting region present immediately outside R-OSF can also be checked through a similar method.
Further, the R-OSF position can also be checked by using the outside diameter of a circular region where laser scattering tomography defects are detected as a reference when a wafer processed from an as-grown single crystal is subjected to infrared scattering tomography to measure the laser scattering tomography defects. Furthermore, the density of dislocation clusters is observed using an optical microscope through the so-called xe2x80x9cSecco etchingxe2x80x9d in which the surface of a specimen wafer is etched using a Secco solution.
Owing to the recent trends not only toward low-temperature processing during device production to get rid of unsatisfactory effects of OSF easily occurring in high-temperature processes, but also toward lower oxygen contents in crystals, R-OSF are not considered so serious a problem as a factor for deteriorating device characteristics. On the other hand, of the grown-in defects, both laser scattering tomography defects and dislocation clusters are factors for deteriorating device characteristics, and thus it is more important to reduce the density of these grown-in defects in the in-plane area of a wafer. While the grown-in defects are less dense at the previously described denuded zones adjacent to R-OSF, such zones are limited to very narrow regions.
Various methods have so far been proposed to reduce the density of grown-in defects in the in-plane area of a wafer. For example, Japanese Unexamined Patent Application Laid-Open No. 8-330316(1996) proposes a method in which only the outer region of R-OSF is expanded into the whole in-plane area of a crystal without causing dislocation clusters to occur by controlling the pulling rate and the temperature gradients within the crystal during single crystal growth. However, according to the proposed method, both extremely limited in-plane temperature gradient and pulling conditions must be satisfied at the same time, and thus new improvements must be made in growing silicon single crystals for which larger diameter and mass production are called.
Next, Japanese Unexamined Patent Application Laid-Open No. 7-257991(1995) and Journal of Crystal Growth 151 (1995, pp. 273-277) disclose methods in which the temperature gradients in the direction of the pulling shaft of a single crystal are increased, so that R-OSF can disappear into the inside of the crystal under high-speed pulling conditions, and thus the outer region of R-OSF can be expanded into the whole in-plane area of the crystal. However, the methods disclosed by these publications have given no considerations to the distribution of temperature gradients in the in-plane area of the crystal, i.e., the uniformity of a temperature distribution in the in-plane area of a wafer and to the in-plane homogenization of introduced point defects. In other words, no considerations are given to means for reducing grown-in defects in the in-plane area of a wafer, and thus, even if only R-OSF are shrunk inward, dislocation clusters do remain in the in-plane area of the wafer as in conventional crystals. Therefore, the methods disclosed in these publications are not successful either in processing wafers having a lower density of grown-in defects.
This invention has been made in view of the above-described conventional problems over crystal defects. It is, therefore, an object of the invention to provide a high-quality silicon single crystal in which regions free of grown-in defects such as laser scattering tomography defects and dislocation clusters can be expanded into the in-plane area of a wafer by controlling the position and width of R-OSF while adjusting single crystal growing conditions. A further object of the invention is to provide a high-quality silicon single crystal that can be grown into a large diameter and a long size. This invention that has been accomplished to achieve the above objects has as its gist the following first to fifth high-quality silicon single crystals and methods of producing such high-quality silicon single crystals.
1. First High-Quality Silicon Single Crystal
(1) A high-quality silicon single crystal grown by a CZ method, characterized in that the width of R-OSF exceeds 8% of the radius of the grown crystal and dislocation clusters are absent; or
(2) A high-quality silicon single crystal grown by a CZ method, characterized in that the width of R-OSF exceeds 8% of the radius of the grown crystal, the inside diameter of the R-OSF is within a range of 0-80% of the diameter of the grown crystal, and dislocation clusters are present at a low density or absent.
2. Second High-Quality Silicon Single Crystal
(1) A high-quality silicon single crystal grown by a CZ method, characterized in that the outside diameter and the inside diameter of a region where ring-like extending oxidation-induced staking faults occur are within a range of 0-80% and within a range of 0-33% of the diameter of said grown crystal, respectively, and dislocation clusters are absent;
(2) A high-quality silicon single crystal grown by a CZ method, characterized in that the inside diameter of a ring-like oxygen precipitation promoting region is within a range of 0-80% of the diameter of said grown crystal, the inside diameter of a region where ring-like extending oxidation-induced stacking faults occur which is in the inner side of said oxygen precipitation promoting region is within a range of 0-33% of the diameter of said grown crystal, and dislocation clusters are absent; or
(3) A high-quality silicon single crystal grown by a CZ method, characterized in that the outside diameter and the inside diameter of a ring-like region where the amount of oxygen precipitates is small are within a range of 0-80% and within a range of 0-33% of the diameter of said grown crystal, respectively, and dislocation clusters are absent.
3. Third High-Quality Silicon Single Crystal and Method of Producing the Same
(1) A high-quality silicon single crystal grown under such a condition that the crystal stays in a temperature range of 1250xc2x0 C.-1000xc2x0 C. for 7 hours or more when pulled by a CZ method, characterized in that the outside diameter of R-OSF is within a range of 0-60% of the diameter of the grown crystal, and a method of producing this single crystal.
(2) A high-quality silicon single crystal grown under such a condition that the crystal stays in a temperature range of 1250xc2x0 C.-1000xc2x0 C. for 7 hours or more when pulled by a CZ method, characterized in that the inside diameter or the outside diameter of an oxygen precipitation promoting region is within a range of 0-60% of the diameter of the grown crystal, and a method of producing this single crystal.
(3) A high-quality silicon single crystal grown under such a condition that the crystal stays in a temperature range of 1250xc2x0 C.-1000xc2x0 C. for 7 hours or more when pulled by a CZ method, characterized in that the outside diameter of a ring-like region where the amount of oxygen precipitates is small is within a range of 0-60% of the diameter of the grown crystal, and a method of producing this single crystal.
(4) A high-quality silicon single crystal grown under such a condition that the crystal stays in a temperature range of 1250xc2x0 C.-1000xc2x0 C. for 7 hours or more when pulled by a CZ method, characterized in that the outside diameter of a circular region where laser scattering tomography defects are detected is within a range of 0-60% of the diameter of the grown crystal, and a method of producing this single crystal.
4. Fourth High-Quality Silicon Single Crystal and Method of Producing the Same
(1) A high-quality silicon single crystal grown under such a condition that a temperature gradient in the vertical direction parallel with a pulling shaft of the crystal is smaller at the outer edge than at the center and is 2.6xc2x0 C./mm or more at the center when the crystal stays in a temperature range of its solidifying point to 1250xc2x0 C. while pulled by a CZ method, characterized in that the outside diameter of R-OSF is within a range of 0-60% of the diameter of the grown crystal, and a method of producing this single crystal; and
(2) A method of producing a high-quality silicon single crystal characterized in that the single crystal is grown under such conditions that a temperature gradient in the vertical direction parallel with a pulling shaft of the crystal is smaller at the outer edge than at the center and is 2.6xc2x0 C./mm or more at the center when the crystal stays a temperature range of its solidifying point to 1250xc2x0 C. during growth, and that the outside diameter of R-OSF is within a range of 0-60% of the diameter of the grown crystal.
5. Fifth High-Quality Silicon Single Crystal and Method of Producing the Same
(1) A high-quality silicon single crystal grown in such a state that the shape of a solid-melt interface between the single crystal and a melt is flat or upwardly convex when pulled by a CZ method, characterized in that the outside diameter of R-OSF is within a range of 0-60% of the diameter of the grown crystal; and
(2) A method of producing a high-quality silicon single crystal characterized in that the single crystal is pulled in such a state that the shape of a solid-melt interface between the single crystal being grown and a melt is flat or upwardly convex at such a low rate as to allow the outside diameter of R-OSF occurring in the single crystal to be within a range of 0-60% of the diameter of the crystal. In this producing method, it is desirable to set the rotating speed of a crucible at 5 rpm or less, or/and the rotating speed of the single crystal at 13 rpm or more.
In this invention, the distribution of each type of defect may be detected through X-ray topography after immersing an as-grown wafer or specimen into an aqueous solution of copper nitrate to thereby deposit Cu thereon and then heat-treating it for 20 minutes at 900xc2x0 C. Further, when the oxygen concentration is decreased, the distribution of OSF may not be observed satisfactorily in some cases under this condition. In such cases, X-ray topography may be used after charging an as-grown wafer or specimen into a furnace whose temperature has reached about 650xc2x0 C., heating it up to 900xc2x0 C. at a rate of 5xc2x0 C./min, soaking it for 20 hours, thereafter heating it to 1000xc2x0 C. at a rate of 10xc2x0 C./min, and soaking it for 10 hours at that temperature. The density of dislocation clusters is detected by subjecting the surface of the wafer or specimen to Secco etching and observing its defects using an optical microscope. Further, laser scattering tomography defects are detected through laser scattering tomograpy.